Communication system with fast control traffic

ABSTRACT

A method and system for conducting rapid control traffic in a time division multiple access (TDMA) communication system comprises a base station communicating with a plurality of user stations in assigned time slots of a time frame. For bearer traffic, time slots are assigned to particular user stations for an extended duration. In unassigned time slots, the base station transmits a general polling message indicating availability of the time slot. A user station desiring to hand off communication from one base station to another uses multiple available time slots at the target base station for exchanging control traffic messages with the target base station. The next available time slot is indicated by a slot pointer in the header of each general polling message to facilitate rapid exchange of control traffic messages. During handover, the user station may establish a new link with the target base station before relinquishing the existing communication link with the old base station.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation application of U.S.application Ser. No. 09/795,005, filed on Feb. 26, 2001, pending, whichis a continuation of U.S. application Ser. No. 09/122,565, filed on Jul.24, 1998, issued as U.S. Pat. No. 6,301,242 and a continuation-in-partof application Ser. No. 09/407,008, filed Sep. 28, 1999, pending, whichis a continuation of U.S. application Ser. No. 08/668,483, filed Jun.21, 1996, which issued as U.S. Pat. No. 6,005,856, which is acontinuation-in-part of U.S. application Ser. No. 08/284,053, filed Aug.1, 1994, issued as U.S. Pat. No. 6,088,590, which is acontinuation-in-part of U.S. application Ser. No. 08/215,306, filed Mar.21, 1994, abandoned, which is a continuation-in-part of U.S. applicationSer. No. 08/146,496, filed Nov. 1, 1993, abandoned.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The field of the present invention relates to wirelesscommunication and, more particularly, to communication protocols forcontrol traffic in a wireless communication system.

[0004] 2. Description of Related Art

[0005] A mobile communication system may generally comprise a set of“user stations”, typically mobile and the endpoints of a communicationpath, and a set of “base stations”, typically stationary and theintermediaries by which a communication path to a user station may beestablished or maintained. A group of base stations may be connected toa base station controller, or a cluster controller, which can in turn beconnected to a local public telephone network through, for example, amobile switching center.

[0006] It is generally desirable in a mobile communication system toachieve the greatest possible user traffic capacity at a base station,so that fewer base stations need to be deployed in order to serve userdemands. One technique used to allow a base station to communicate withmultiple user stations is use of time division multiple access (TDMA).In a particular TDMA system, for example, a time frame is divided into aplurality of smaller, time units, or time slots, and transmissions fromthe base station and from the user stations are separated in time so asto avoid collisions. In addition to separating transmissions in time,transmissions may also be distinguished by using different assignedfrequencies, thereby resulting in a frequency division multiple access(FDMA) system. Furthermore, transmissions may be encoded using spreadspectrum techniques, and different cells in a mobile communicationsystem may be assigned different spread spectrum codes, therebydifferentiating transmissions through code division multiple access(CDMA).

[0007] Generally, in order to carry out communication between a basestation and a user station, a communication link must first beestablished. Establishment of the communication link can be difficult ina spread-spectrum communication system, due to the length of timetypically required to synchronize the transmitter and the receiver.Establishment of the communication link and/or handing off can be moredifficult in a TDMA system in which spread spectrum is used, due to theamount of time usually necessary to synchronize the transmitter andreceiver, especially where the amount of time available forsynchronization within a user station's time slot is relatively brief.

[0008] Within a mobile communication system, a protocol generallydefines how communication is to be initially established between a basestation and a user station. The protocol may further define when and howa handoff may be conducted as a user station leaves the service area or“cell” of one base station and enters the service area of another basestation. Messages exchanged between a base station and user station forthe purposes of establishing or maintaining a connection, or for handingoff communication, generally can be referred to as control traffic orsignaling traffic. Messages carrying data to be conveyed between theendpoints of a call are generally referred to as bearer trafficmessages.

[0009] Initial communication between a user station and a base stationcan be established either when the user station seeks to initiatecommunication with a base station (for example, attempting to initiate atelephone call), or when the base station attempts to complete a call tothe user station (for example, where the user station is paged). In manyconventional mobile communication systems, a dedicated control channelis used to assist mobile stations in establishing communication.According to this technique, the mobile station first communicates overthe control channel when establishing communication. The base stationthen assigns to the mobile station a “permanent” communication channelfor exchanging bearer traffic messages.

[0010] In at least one mobile communication system, however, a userstation can establish initial communication using the same channel usedfor transmitting bearer traffic. For example, a system in which a userstation can establish communication by exchanging control trafficmessages in a particular communication channel (e.g., a time slot-of atime frame), and thereafter use the same channel (time slot) for bearertraffic, is described in U.S. Patent Application Ser. No. 08/284,053filed Aug. 1, 1994, which is assigned to the assignee of the presentinvention, and hereby incorporated by reference as if set forth fullyherein.

[0011] The exchange of control traffic messages may also occur during ahandoff of a user station from one base station to another, usually asthe user station moves between service areas. Typically, in the largemajority of conventional mobile communication systems, handoffs arecarried out under the direction of the base station and/or a mobilitycontrol center connected to the base station. When a communication linkstarts to break down, the base station requests a transfer of an ongoingcall to a nearby base station, which becomes the target for handoff. Thetarget base station may be selected according to criteria developed atthe base station, the user station, or both. A control channel (whichmay be the same dedicated control channel as used for establishingcommunication, where provided) may be used for the purpose of assistingthe mobile station with the handoff.

[0012] In some mobile communication systems, the user station plays alarger role in handoff. An example of such a system is generallydescribed in U.S. patent application Ser. No. 08/284,053, previouslyincorporated herein by reference. In at least one embodiment disclosedtherein, the user station not only determines when to hand off, but alsotakes steps to initiate a hand off from its current base station to adifferent base station.

[0013] It is generally desirable in mobile communication systems toallow the rapid establishment of communication links between mobilestations and base stations, and rapid handoff between base stations,without errors and without inadvertently dropping the call or losing acommunication link. This type of capability would tend to imply the needfor devoting potentially significant resources (i.e., communicationchannels and processing speed and power) to handle link establishmentand handoff. Because the communication environment can be unstable andmultiple users may need to be serviced at the same time, a mobilecommunication system is preferably capable of handling multiple servicerequests for link establishment or handoff, and doing so quickly andwithout errors or dropped calls.

[0014] At the same time, resources available for handling controltraffic messages are usually limited, sometimes severely so, in partbecause control traffic resources generally must compete against bearertraffic resources. Thus, resources dedicated to control traffic reducethe overall resources available for handling data or bearer traffic, andvice versa. By setting aside resources (such as a dedicated controlchannel or multiple such channels) for servicing control trafficdemands, the base station's user capacity can be adversely impacted. Asa result, a greater number of base stations may need to be deployed toservice a given number of expected users.

[0015] It would therefore be advantageous to provide a communicationsystem having a rapid and reliable means for establishing acommunication link between a base station and a user station. It wouldfurther be advantageous to provide a communication protocol enablingrapid handoffs and control traffic functions, and which is particularlysuited to use in a time division multiple access environment. It wouldfurther be advantageous to provide a communication protocol having afast handoff and control traffic capability well suited to the demandsof spread spectrum communication.

SUMMARY OF THE INVENTION

[0016] In one aspect of the present invention, a method and system forhanding off communication between base stations in a mobilecommunication system is provided. In a preferred embodiment of theinvention, a mobile station communicates with a base station using atime division multiple access (TDMA) and/or time division duplex (TDD)technique. In such an embodiment, a continuous sequence of time framesis generated, with each time frame comprising a plurality of time slots.The base station can communicate with a plurality of user stations (someor all of which may be mobile stations), one in each time slot. A mobilestation desiring to hand off exchanges a plurality of control trafficmessages with a second base station to establish communication in adifferent time slot with the second base station. The mobile stationthen releases the communication channel with the first base station andrequests, through the second base station, the transfer of the call tothe second base station.

[0017] In a preferred embodiment of the present invention, a mobilestation transmits and/or receives a plurality of control trafficmessages in multiple time slots of one or more time frames with thesecond (target) base station while in the process of handing offcommunication to the target base station, or performing other controltraffic signaling. The second base station provides an indication to themobile station of the next available time slot for control traffic, and,if desired, can temporarily assign additional time slots to the mobilestation during handoff, or other control traffic signaling.

[0018] In another aspect of the present invention, a method and systemfor establishing communication and handing off communication in a TDMAand/or TDD communication is provided. In one embodiment, the basestation transmits a general poll message in each available time slot toindicate availability of the time slot. To establish communication in anavailable time slot, a user station responds to the general poll messagewith a general poll response. The base station then follows with aspecific poll message. The user station responds with a specific pollresponse. Normal traffic communication may thereafter be conducted overan established communication link. During normal traffic communication,in one embodiment, each user station transmits information to the basestation during an initial portion of an assigned time slot, and eachuser station receives information from the base station during a latterportion of the same assigned time slot.

[0019] Handover between base stations may be carried out by establishinga new communication link with a new base station, while maintaining anold communication link with an original base station until the newcommunication link is fully established. The new communication link maybe established in the same manner as the original link—that is, by usingthe same handshaking technique involving a general poll, generalresponse, specific poll, and specific response messages.

[0020] In another aspect of the invention, a slot pointer informationelement within a general polling message provides an indication of thelocation of the next available time slot for communication. The slotpointer may be a numerical value relative to the current time slot. Aspart of a specific polling message, the slot pointer information elementprovides an assignment of the time slot channel to be used for futurecommunication by the user station presently in the process ofestablishing communication. The slot pointer may be used to performrapid handover by allowing the use of multiple time slots within a timeframe for control traffic.

[0021] In another embodiment, virtual time slots are defined as part ofthe timing structure. As used herein, a virtual time slot is generally atime slot assigned to the same user station with two transmissionintervals non-adjacent in time. For example, a virtual time slot may bea time slot in which a forward link transmission and a reverse linktransmission for a particular user station are separated bytransmissions to or from one or more other user stations. In a preferredsystem in which each physical time slot has a user transmission intervaland a base transmission interval, a user station may therefore transmita user message to the base station during a user transmission intervalof a first physical time slot, and receive a base message from the basestation during a base transmission interval of a second, subsequentphysical time slot. In a particular embodiment, a virtual slot field inthe header of the general polling message indicates whether or notvirtual time slots are provided, thereby enabling operation in either oftwo modes, one using virtual time slots and the other not using virtualtime slots.

[0022] A method and system for establishing and maintaining spreadspectrum communication is disclosed with respect to a preferredembodiment wherein data symbols are encoded using an M-ary directsequence spread spectrum communication technique. Further variations anddetails of the above embodiments are also described herein and/ordepicted in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagrammatic representation of a cellularcommunication system.

[0024]FIG. 1A is a diagram of an arrangement of cells in a wirelesscommunication system showing an exemplary code and frequency reusepattern.

[0025]FIG. 2 is a diagram of one embodiment of a communication system.

[0026]FIG. 2A is a block diagram of another embodiment of acommunication system, using a GSM-based network interconnection.

[0027]FIG. 3 is a diagram of a time frame divided into time slots.

[0028]FIG. 4 is a diagram illustrating a protocol for establishing acommunication link between a base station and a user station.

[0029]FIG. 4A is a message flow diagram corresponding to FIG. 4.

[0030]FIG. 5A is a diagram of a preferred time slot structure.

[0031]FIGS. 5B and 5C are diagrams of a base station transmit data timeframe structure and a user station transmit data time frame structure,respectively.

[0032]FIG. 6 is a diagram of a time frame structure in accordance withanother embodiment of the invention showing a time frame divided intovirtual time slots.

[0033] FIGS. 7A-7C are diagrams of polling message formats.

[0034]FIGS. 8A and 8B are diagrams of message header formats.

[0035]FIG. 9 is a message flow diagram illustrating call originationfrom a user station.

[0036]FIG. 10 is a message flow diagram illustrating call termination atthe user station.

[0037] FIGS. 11A-11C are message flow diagrams illustrating a handoverof a mobile call between two base stations within a cluster.

[0038]FIGS. 12A and 12B are message flow diagrams illustrating ahandover of a mobile call between two base stations located in differentclusters.

[0039]FIGS. 13A and 13B are diagrams of a base station data packet and auser station data packet, respectively.

[0040]FIGS. 14A and 14B are timing diagrams showing a time frame andtime slot structure in a linear representation and loop representation,respectively.

[0041]FIG. 15 is a diagram of a series of consecutive time framesshowing utilization of a particular time slot over a sequence of timeframes.

[0042]FIGS. 16A and 16B are timing diagrams of mobile stationtransmissions and base station transmissions, respectively, within aparticular polling loop of the type shown in FIG. 14B, wherein symmetrictime slots are used.

[0043]FIGS. 17A and 17B are timing diagrams of mobile stationtransmissions and base station transmissions, respectively, within aparticular polling loop of the type shown in FIG. 14B, whereinasymmetric time slots are used.

[0044]FIGS. 18A and 18B are timing diagrams showing multiple time slotsutilized for carrying out control traffic operations.

[0045]FIG. 19 is a block diagram of a communication system illustratinginter-cluster and intra-cluster handoffs.

[0046]FIG. 20 is a block diagram of a transmitter and a receiver in aspread spectrum communication system.

[0047]FIG. 21 is a diagram illustrating a preferred system protocolarchitecture.

[0048]FIG. 22 is a call flow diagram of a call release initiated by auser station.

[0049]FIG. 23 is a call flow diagram of a call release initiated by thenetwork.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050]FIG. 1 is a diagram of a pattern of cells for a multiple-accesswireless communication system 101. The wireless communication system 101of FIG. 1 includes a plurality of cells 103, each with a base station104, typically located at the center of the cell 103. A plurality ofuser stations 102, some or all of which may be mobile, communicate withthe base stations 104 to place and receive calls. Each station (both thebase stations 104 and the user stations 102) generally comprises areceiver and a transmitter.

[0051] A control station 105 may also be provided (comprising a receiverand a transmitter) to manage the resources of the system 101. Thecontrol station 105 (which may comprise a “base station controller” asdescribed later herein) may assign the base station 104 and userstations 102 in each cell 103 a spread-spectrum code or a set of spreadspectrum codes for modulating radio signal communication in that cell103. (Alternatively, a spread spectrum code or set of spread spectrumcodes may be pre-assigned to a cell 103.) The resulting spread spectrumsignals are generally spread across a bandwidth exceeding the bandwidthnecessary to transmit the data, hence referred to by the term “spreadspectrum.” Accordingly, radio signals used in a cell 103 are preferablyspread across a bandwidth sufficiently wide that both base station 104and user stations 102 in an adjacent cell 103 can distinguishcommunication which originates in the first cell 103 from communicationwhich originates in the adjacent cell 106.

[0052]FIG. 2 is a block diagram of a communication system architectureutilized in a preferred embodiment of the present invention. The FIG. 2communication system comprises a plurality of base stations 104 forcommunicating with a plurality of user stations 102. The base stations104 and user stations 102 may operate in a personal communicationssystem (PCS), such as may be authorized under rules prescribed by theFederal Communications Commission (FCC).

[0053] Each base station 104 may be coupled to a base station controller105 by any of a variety of communication paths 109. The communicationpaths 109 may each comprise one or more communication links 118. Eachcommunication link 118 may include a coaxial cable, a fiber optic cable,a digital radio link, or a telephone line.

[0054] Each base station controller 105 may also be connected to one ormore communication networks 126, such as a public switched telephonenetwork (PSTN) or personal communication system switching center (PCSC).Each base station controller 105 is connected to a communication network126 by means of one or more communication paths 108, each of which mayinclude a coaxial cable, a fiber optic cable, a digital radio link, or atelephone line.

[0055] The FIG. 2 communication system also may include one or more“intelligent” base stations 107 which connect directly to acommunication network 126 without interfacing through a base stationcontroller 105. The intelligent base stations 107 may therefore bypassthe base station controller 105 for local handoffs and switching of userstations 102, and instead perform these functions directly over thenetwork 126.

[0056] In operation each base station 104 formats and sends digitalinformation to its respective base station controller 105 (or directlyto the network 126 in the case of an intelligent base station 107). Thebase station controllers 105 receive inputs from multiple base stations104, assist handoffs between base stations 104, and convert and formatchannel information and signaling information for delivery to thenetwork 126. The base station controllers 105 may also manage a localcache visitor location register (VLR) database, and may support basicoperation, administration and management functions such as billing,monitoring and testing. Each base station controller 105, under controlof the network 126, may manage local registration and verification ofits associated base stations 104 and may provide updates to the network126 regarding the status of the base stations 104.

[0057] The network 126 connects to the base station controllers 105 forcall delivery and outgoing calls. Intelligent base stations 107 may useISDN messaging for registration, call delivery and handoff over a publictelephone switch. The intelligent base station 107 may have all thegeneral capabilities of a base station 104 but further incorporate abasic rate ISDN (BRI) card, additional intelligence and local vocoding.

[0058] The communication system may also be based on a GSM networkinterconnection. FIG. 2A is a diagram of a communication systemarchitecture showing such an interconnection. In the communicationsystem shown in FIG. 2A, the base stations 104 may connect to a GSMmobile switching center 112 through a GSM “A” interface. The “A”interface may be incorporated in base station controllers 105 and inintelligent base stations 107. Features and functionality of GSM may bepassed to and from the base stations 104 over the “A” interface in amanner that is transparent to the end user (i.e., user stations 102).The GSM mobile switching center 112 may connect to a PSTN or to othernetworks, as indicated in FIG. 2A.

[0059] The system may also interconnect to cable television distributionnetworks. In such a system, the base stations 104 may be miniaturized sothat they can be installed inside standard cable TV amplifier boxes.Interfacing may be carried out using analog remote antenna systems anddigital transport mechanisms. For example, T1 and fractional T1 (“FT1”)digital multiplexer outputs from the cable TV network may be used forinterfacing, and basic rate (BRI) ISDN links may be used to transportdigital channels.

[0060]FIG. 1A is a diagram of a preferred cellular environment in whichthe invention may operate. According to FIG. 1A, a geographical region201 is divided into a plurality of cells 103. Associated with each cell103 is an assigned frequency and an assigned spread spectrum code.Preferably, three different frequencies (or frequency groups) F1, F2 andF3 are assigned in such a manner that no two adjacent cells have thesame assigned frequency (or frequency group) F1, F2 or F3, therebyminimizing RF interference between adjacent cells. The frequencies maybe assigned on a “permanent” basis, or else dynamically through thenetwork.

[0061] To further reduce the possibility of intercell RF interference,different near-orthogonal spread spectrum codes C1 through C7 areassigned as shown in a repeating pattern overlapping the frequency reusepattern. Although a repeating pattern of seven spread spectrum codes C1through C7 is preferred, a pattern involving other numbers of spreadspectrum codes may be suitable depending upon the particularapplication. As with frequencies used in the cells 103, spread spectrumcodes may be assigned on a “permanent” basis or else dynamically throughthe network. Further information regarding a suitable cellularenvironment for operation of the invention may be found in U.S. Pat. No.5,402,413, assigned to the assignee of the present invention, and herebyincorporated by reference as if fully set forth herein.

[0062] The use of spread spectrum for carrier modulation permits afrequency reuse factor of N=3 for allocating different carrierfrequencies F1, F2 and F3 to adjacent cells 103. Interference betweencells 103 using the same carrier frequency F1, F2 or F3 is reduced bythe propagation loss due to the distance separating the cells 103 (i.e.,any two cells 103 using the same frequency F1, F2 or F3 are separated byat least one intervening cell 103, as shown in FIG. 1A), and also by thespread spectrum processing gain obtained by the use of near-orthogonalspreading codes.

[0063] Further details regarding an exemplary cellular pattern aredescribed in, e.g., U.S. Pat. No. 5,402,413 referred to above.

[0064] A preferred embodiment of the invention achieves multiple accesscommunication by using a time frame divided into multiple time slots,i.e., time division multiple access (TDMA). FIG. 3 is a diagram showinga timing structure for a particular TDMA system. According to the timingstructure of FIG. 3, communication over time is broken into a continuousseries of time frames 301. A single complete time frame 301 is shownalong a timeline 310 in FIG. 3; similar time frames are assumed toprecede and follow time frame 301 in a continuous pattern along thetimeline 310.

[0065] Time frame 301 is divided into a plurality of time slots 302numbered consecutively TS1, TS2 . . . TSN, each of which may supportduplex communication with a user station 102. Time frame 301 may bethought of as a “polling loop” or a time loop, as depicted in FIG. 3,whereby user stations 102 are communicated with sequentially over thetime frame 301 in a manner analogous to polling, each user station 102transmitting and receiving messages in its designated time slot 302. Inthe FIG. 3 embodiment, each time slot 302 comprises a user transmissioninterval 305, wherein a user station 102 transmits a user-to-basemessage to the base station 104, and a base transmission interval 306,wherein the base station 104 transmits a base-to-user message to theuser station 102. Communication in time slots 302 may be interleaved,such that user stations 102 transmit in one physical time slot 302 butreceive in a different physical time slot 302.

[0066] In an exemplary TDMA communication system, time frames 301 areeach in the neighborhood of 20 milliseconds in duration, and each timeframe 301 comprises sixteen time slots 302 or, alternatively, eight timeslots 302 to support extended range through increased guard times.

[0067] In some embodiments, a user station 102 may communicate in morethan one time slot 302 in each time frame 301, so as to support anincreased data rate. Similarly, in some embodiments, a user station 102may periodically skip time frames 301 and communicate in some subset ofall time frames 301 (e.g., every other time frame 301, or every fourthtime frame 301), so as to support a reduced data rate where a full speedcommunication link is not necessary. Further information about anexemplary TDMA system supporting variable data rates as described abovemay be found in copending U.S. patent application Ser. No. 08/284,053filed Aug. 1, 1994, previously incorporated herein by reference.

[0068]FIG. 6 is a diagram of a timing structure employing virtual timeslots, each of which generally comprises a duplex pair (i.e., oneforward link and one reverse link).

[0069] In FIG. 6, similar to FIG. 3, communication over time is brokeninto a continuous series of time frames 601. A single complete timeframe 601 is shown along a timeline 610 in FIG. 6; similar time framesare assumed to precede and follow time frame 601 in a continuous patternalong the timeline 610.

[0070] Time frame 601 is divided into a plurality of physical time slots602 numbered consecutively TS1′, TS2′ . . . TSN′. Each physical timeslot. 602 comprises a user transmission interval 605 wherein a userstation 102 transmits a user-to-base message to the base station 104,and a base transmission interval 606 wherein the base station 104transmits a base-to-user message to a user station 102, which could be adifferent user station 102 than transmitted to the base station 104 inthe same physical time slot 602. Using virtual time slots, communicationin physical time slots 602 may be interleaved, such that a user station102 transmits in one physical time slot 602 but receives in a differentphysical time slot 602. The user transmission interval 605 and basetransmission interval 606 which define the forward link and reverse linktransmissions for a given user station 102 (and which are generallylocated in different physical time slots 602, as depicted in FIG. 6) arecollectively referred to as a “virtual time slot.”

[0071] An exemplary virtual time slot 618 is shown in FIG. 6, associatedwith a particular user station 102 (e.g., user station MS2). The virtualtime slot 618 comprises two message transmission intervals, one in eachof two physical time slots 602 a and 602 b. Virtual time slot 618 has auser transmission interval 605 a in the first physical time slot 602 a,and a base transmission interval 606 b in the second physical time slot602 b. Between the user transmission interval 605 a and the basetransmission interval 606 b of the virtual time slot 618, the basestation 104 transmits in a base transmission interval 606 a of the firstphysical time slot 602 a (e.g., to a second user station 102, such asuser station MS1), and another user station 102 (e.g., a third userstation 102, such as user station MS3) transmits in a user transmissioninterval 605 b to the base station 104. In this manner, transmissions toand from the base station 104 are interleaved.

[0072] Time frame 601 may be thought of as a “polling loop” or a timeloop, similar to time frame 301 of the FIG. 3 embodiment, whereby userstations 102 are communicated with sequentially over the time frame 601in a manner analogous to polling, each user station 102 transmitting andreceiving messages in its designated virtual time slot 618. The virtualtime slots 618 of FIG. 6, however, are not necessarily identical to thephysical time slots 602. An advantage of the FIG. 6 timing structure isthat it may allow extended time for the base station 104 to processchannel characterization data as received from the user station 102.

[0073] In an exemplary TDMA communication system, time frames 601 areeach 20 milliseconds in duration, and each time frame 601 comprisessixteen time slots 602 or, alternatively, eight time slots 602 tosupport extended range through increased guard times.

[0074] Further details regarding time frame structures (includingvirtual time slots) may be found in copending U.S. patent applicationSer. No. 08/668,483 filed Jun. 21, 1996, hereby incorporated byreference as if set forth fully herein.

[0075] In some embodiments, a user station 102 may communicate in morethan one virtual time slot 618 in each time frame 601, so as to supportan increased data rate. Similarly, in some embodiments, a user station102 may periodically skip time frames 601 and communicate in some subsetof all time frames 601 (e.g., every other time frame 601, or everyfourth time frame 601), so as to support a reduced data rate where afull speed communication link is not necessary.

[0076] Communication between a user station 102 and a base station 104is established in one embodiment by a response from a user station 102to a general polling message sent from the base station 104 during anavailable time slot 302. This process is described in more detail withreference to FIG. 4, which illustrates a protocol for establishment of aspread spectrum communication link in, e.g., the FIG. 3 communicationsystem. A communication link may be established in an analogous mannerfor the FIG. 6 embodiment.

[0077] In the FIG. 4 protocol, a general poll message 401 is transmittedby the base station 104 in some or all of the time slots 302 which areavailable for communication. A user station 102 may monitortransmissions from a base station 104 and ascertain available time slots302 by receiving general poll messages 401 in those time slots 302.

[0078] A user station 102 may “acquire” a base station 104 by a sequenceof handshaking steps. At a general poll step 407, the base station 104transmits its general poll message 401 during an unoccupied time slot302. The user station 102 receives the general poll message 401 and, ifit was received without error, transmits a general poll response 404 tothe base station 104 in the same time slot 302 of the following timeframe 301 (or in a different time slot, as explained hereafter). Thegeneral poll message 401 preferably comprises a field for a base ID 408b, which may be 32 bits long (for example), and which may be stored orotherwise recorded by the user station 102. Similarly, the general pollresponse 404 preferably comprises a field for a user ID 409, which maybe 32 bits long (for example), and which may be stored or otherwiserecorded by the base station 104.

[0079] Upon receiving a general poll response 404, at a specific pollstep 410 the base station 104 transmits a specific poll message 402comprising (among other things) the user ID 409 which had beenpreviously received by the base station 104 as part of the general pollresponse 404. The user station 102 receives the specific poll message402 and, if it was received without error and with the same user ID 409,transmits its specific poll response 405 to the base station 104 in thesame time slot 302 of the following time frame 301 (or in a differenttime slot, as explained further herein). The specific poll response 405comprises the same user ID 409 as the general poll response 404.

[0080] In a particular embodiment, the specific poll response 405 may beeliminated as redundant. The user station 102 may, in such a case,follow the specific poll message 402 with a user traffic message 406.

[0081] Upon receiving a specific poll response 405 comprising in a userID 409 which matches that of the general poll response 404, at alink-established step 411 the base station 104 may transmit a trafficmessage 403. At this point, the base station 104 and user station 102have established a communication link 412. The base station 104 mayconnect a call through the communication channel, and the user station102 may begin normal operation on a telephone network (e.g., the userstation 102 may receive a dial tone, dial a number, make a telephoneconnection, and perform other telephone operations). The base station104 and user station 102 may exchange traffic messages 403 and 406,until the communication link 412 is voluntarily terminated, until faultycommunication prompts the user station 102 to re-acquire the basestation 104, or until handoff of the user station 102 to another basestation 104.

[0082]FIG. 4A illustrates a similar exchange of messages in a messageflow diagram format, whereby a user station 102 establishescommunication with a base station 104.

[0083] Should more than one user station 102 respond to the same generalpoll message 401, the base station 104 may intentionally fail to respondwith a specific poll message 402. The lack of response from the basestation 104 signals the involved user stations 102 to back off for acalculated time interval before attempting to acquire the same basestation 104 using the general poll message 401 and general poll response404 protocol. The back-off time may be based upon the user ID 409, andtherefore each user station 102 will back off for a different length oftime to prevent future collisions, in a manner similar to that specifiedby IEEE Standard 802.3.

[0084] When an incoming telephone call is received at a base station 104at an incoming-call step 413, the base station 104 skips the generalpoll message 401 and general poll response 404 and moves directly to thespecific poll step 410. The base station 104 transmits a specific pollmessage 402 with the user ID 409 of the indicated recipient user station102 on an available time slot 302. As further described herein, eachuser station 102 listens regularly for the specific poll message 402 soas to receive the specific poll message 402 within a predetermined timeafter it is transmitted. When the specific poll message 402 is received,the user station 102 compares the user ID 409 in the message with itsown user ID, and if they match, continues with the link-established step411. The base station 104 may thereby establish a communication link 412with any user station 102 within communication range.

[0085] Further details regarding means for establishing communication(particularly spread spectrum communication) in a TDMA system may befound in copending U.S. Pat. No. 5,455,822 and in copending U.S. patentapplication Ser. No. 08/284,053 filed Aug. 1, 1994, both of which arehereby incorporated by reference as if fully set forth herein.

[0086] In a preferred embodiment, the general poll message 401 comprisesa next slot pointer (contained in a next slot pointer field 810 shown inand described with respect to FIG. 8A) which indicates the next timeslot 302 (or virtual time slot 618) during which a general poll message401 will be transmitted by the base station 104. In such an embodiment,a user station 102 seeking to establish communication responds to thegeneral poll message 401 in the user transmission interval 305 (or 605)of the time slot 302 (or 618) indicated by the next slot pointer, andnot necessarily in the same time slot of the next time frame 301 (or601). Upon receiving a general response message 404 from the userstation 102 in the time slot indicated by the next slot pointer, thebase station 102 responds with a specific poll message 402. Should morethan one user station 102 respond to a general poll message 401, theappearance of a general poll message 401 (rather than a specific pollmessage 402) in the time slot indicated by the next slot pointer willcause each user station 102 involved to back off for a variable periodof time depending on the user station ID.

[0087] The specific poll message 402 comprises a temporary shorthandidentifier (nickname) specific to the user station 102 and referred toherein as a “correlative ID.” The correlative ID appears in subsequentsignaling messages (in both directions) until the established link isdropped. In response to the specific poll message 402, the user station102 responds with a traffic message in a time slot 302 (or 618) assignedby a next slot pointer in the header of the specific poll message 402.

[0088] Further details of how the next slot pointer (sometimes referredto simply as the slot pointer) is used within preferred embodiments aredescribed below, after a brief description of various time intervalswithin a time slot and basic message structures and formats. Theparticular time intervals, messages structures and formats are meant tobe illustrative and to represent various preferred embodiments fordemonstrating the workings of the invention, and are not meant to limitthe invention to any particular type of message structure or format, orany particular type of, time slot structure.

[0089]FIG. 5A is a diagram of a preferred slot structure, and FIGS. 5Band 5C are diagrams of a base station transmit data frame structure anda user station transmit date frame structure, respectively. In FIG. 5A,a time slot 510 comprises a variable radio delay gap 505, a user stationtransmit frame 515, a base processor gap 525, a guard time 535, a basestation transmit frame 545, and a radar gap 555. Each user stationtransmit frame 515 comprises a user preamble 516, a user preamblesounding gap 519, and a user station transmit data frame 521. Similarly,each base station transmit frame 545 comprises a base preamble 547, abase preamble sounding gap 549, and a base transmit data frame 551.

[0090]FIG. 5B illustrates a preferred message structure for the basestation transmit data frame 551. The message structure of FIG. 5Bcomprises a base header field 553, a base D-channel field 557, a basedata field 559, and a base cyclical redundancy check (CRC) field 561. Ina preferred embodiment, the base header field 553 is 23 bits, the baseD-channel field 557 is 8 bits, the base data field 559 is 192 bits, andthe base CRC field 561 is 16 bits.

[0091]FIG. 5C illustrates a preferred message structure for the userstation transmit data frame 521. The message structure of FIG. 5Ccomprises a user header field 523, a user D-channel field 527, a userdata field 529, and a user CRC field 531. In a preferred embodiment, theuser header field 523 is 17 bits, the user D-channel field 527 is 8bits, the user data field 529 is 192 bits, and the user CRC field 531 is16 bits.

[0092] FIGS. 7A-7C are diagrams of preferred polling message formats.FIG. 7A is a diagram of a general poll message format, such as may beemployed, for example, with general poll message 401 of FIG. 4. As shownin FIG. 7A, a general poll message 701 preferably comprises, in thefollowing sequence, a header field 702, a spare field 703, a zone field70, a base station controller (BSC) ID field 705, a base ID field 706, afacility field 707, a system type field 708, a service provider field709, a slot quality field 710, a forward error correction (FEC) field711, and a frame control word (FCW) field 712. In a preferredembodiment, the header field 702 is 24 bits long, the spare field 703 is16 bits long, the zone field 704 is 40 bits long, the BSC ID field 705is 16 bits long, the base ID field 706 is 32 bits long, the facilityfield 707 is 32 bits long, the system type field 708 is 8 bits long, theservice provider field 709 is 16 bits long, the slot quality field 710is 8 bits long, the FEC field 711 is 32 bits long, and the frame controlword field 712 is 16 bits long, for a total of 240 bits.

[0093] The header field 702 identifies the message type and is describedmore fully with respect to FIG. 8A. The zone field 704 identifies thepaging zone of the specific base station 104. A user station 102 maymove from one base station 104 service area to another in the same zonewithout requiring immediate re-registration. The BSC ID field 705 is asequence uniquely identifying the base station controller 105. The baseID field 706 is a sequence uniquely identifying the base station 104.The facility field 707 describes the services offered by the basestation 104 (e.g., internet access, aggregate data capability, enhancedvoice, etc.). The facility field 707 may include a sub-field indicatingwhat user stations may have access to the channel (e.g., 911 calls only,or user stations 102 with specific access codes). The system type field708 identifies the type of system associated with the base station 104.The service provider field 709 identifies the PCS service provider thatoperates the base station 104 (or, if more than one service provider isavailable at the base station 104, the service provider that currentlyoperates the particular time slot). The slot quality field 710 indicatesthe relative quality of the time slot in terms of interference.Generally, the lower the number, the better the slot quality. The FECfield 711 is used for forward error correction. The FCW field 712 isused for error detection, and in one embodiment comprises a sequence ofbits and/or phase shifts determined according to following algorithm:

[0094] 1. Calculate remainder R1 of a seed polynomial SDP modulo-2divided by a generator polynomial GRP;

[0095] 2. Calculate product P of x¹⁶ and content of the message 701preceding FCW field 710;

[0096] 3. Calculate remainder R2 of the generator polynomial GNPmodulo-2 divided by the product P derived in Step 2;

[0097] 4. Calculate modulo-2 sum S of remainder R1 and remainder R2; and

[0098] 5. Calculate the ones-complement of sum S the result of which istransmitted in the FCW field 710.

[0099] In a preferred embodiment, the seed polynomial SDP is:

x^(k)(x₁₅+x¹⁴+x₁₃+x¹²+x¹¹+x¹⁰+x⁹+x⁸+x⁷+x⁶+x⁵+x⁴+x³+x²+x¹+1)

[0100] and the generator polynomial GRP is:

x¹⁶+x₁₂+x₅+1

[0101]FIG. 7B is a diagram of a specific poll message format (such asmay be employed, for example, with specific poll message 402 of FIG. 4).As shown in FIG. 7B, a specific poll message 720 preferably comprises,in the following sequence, a header field 721, a correlative ID field722, a cause field 723, a personal identifier (PID) field 724, anover-the-air (OTA) map type field 725, an OTA map field 726, a sparefield 727, a slot quality field 728, a forward error correction field729, and an FCW field 730. In a preferred embodiment, the header field721 is 24 bits long, the correlative ID field 722 is 8 bits long, thecause field 723 is 8 bits long, the PID field 724 is 72 bits long, theOTA map type field 725 is 8 bits long, the OTA map field 726 is 32 bitslong, the spare field 727 is 32 bits long, the slot quality field 728 is8 bits long, the FEC field 729 is 32 bits long, and the FCW field 729 is16 bits long, for a total of 240 bits.

[0102] The header field 721, slot quality field 728, FEC field 729, andFCW field 730 are similar to the analogous fields described for FIG. 7A.The correlative ID field 722 is used to temporarily identify one or morechannels (i.e., time slots) as being allocated to a specific userstation 102. A correlative ID number is assigned for the duration of acall connection and is released for reuse by another user station 102 atthe termination of a connection; the correlative ID number may also bechanged during a connection. A specific correlative ID number may bereserved by the base station 104 for broadcast use. The cause field 723indicates the cause of an error occurring during execution of a previoussignaling traffic operation for the particular user station 102.Interpretation of the cause field 723 message may therefore depend uponthe type of signal traffic involved. Possible cause messages include,for example, those indicating that the user station 102 is unregisteredor will not be accepted for registration, or that the call has not beenconnected or cannot be completed. The PID field 724 comprises a personalidentification number which uniquely identifies the subscriber (e.g.,user station 102). The OTA map type field 725 defines the type of map(e.g., superframe, subframe, etc., as defined later herein) that followsin the OTA map field 726. The OTA map field 726 describes the mapping oftime slots relative to a particular user station 102. The format of theOTA map field 726 depends on the map type.

[0103]FIG. 7C is a diagram of a poll response message format (such asmay be employed, for example, with general poll response 404 or specificpoll response 405 of FIG. 4). As shown in FIG. 7C, a poll responsemessage 740 preferably comprises, in the following sequence, a headerfield 741, a first spare field 742, a PID field 743, a service providerfield 744, a class field 745, a user capabilities field 746, a secondspare field 747, an FEC field 748, and an FCW field 749. In a preferredembodiment, the header field 741 is 17 bits long, the first spare field742 is 16 bits long, the PID field 743 is 72 bits long, the serviceprovider field 744 is 16 bits long, the class field 745 is 16 bits long,the user capabilities field 746 is 16 bits long, the second spare field747 is 32 bits long, the FEC field 748 is 32 bits long, and the FCWfield 749 is 16 bits long, for a total of 233 bits.

[0104] The header field 741 identifies the message type and is morefully described in FIG. 8B. The PID field 743, FEC field 748, and FCWfield 746 are similar to the PID field 724, FEC field 729, and FCW field730, respectively, described with respect to FIG. 7B. The serviceprovider field 744 identifies the PCS service provider that the userstation 102 wishes to use. The class field 745 specifies some of theoperational parameters being used by the particular user station 102.The class field 745 may comprise a class type sub-field and a classinformation sub-field. The class type sub-field indicates the userstation class type (e.g., DCS1900 class type, or IS-41 class type,etc.), and may also provide an indication of the power level capabilityof the user station 102. The class information sub-field providesoperational information including, for example, revision level,available encryption algorithms, short message capability, ellipsisnotation and phase-2 error handling capability, power class,continuous/discontinuous transmission, bandwidth (e.g., 20 MHz or 25MHz), and nominal power levels. The class type sub-field may, for aGSM-oriented system, indicate the power level capability of the userstation 102. The user capabilities field 746 identifies the featurespresent in the user station 102 (e.g., whether the user station 102 canreceive a fax or data connection, whether the user station 102 iscapable of ciphering, etc.).

[0105]FIGS. 8A and 8B are diagrams of preferred polling message headerformats. FIG. 8A is a diagram of a polling message header format for abase polling message (such as general poll message 401 or specific pollmessage 402 of FIG. 4). The polling message header 801 comprises abase/mobile indicator (B/M) flag 802, an extended protocol (E) flag 803,a packet type field 804, a power adjustment (PWR) field 805, a symmetryfield 806, a D-channel suppression (DCS) flag 807, a virtual slot (VS)flag 808, a slot or channel utilization (CU) field 809, a slot pointerfield 810, a error check and correct (ARQ) field 811, and a header framecontrol word (HCF) field 812. In a preferred embodiment, the B/Mindicator flag 802, E flag 803, PWR field 805, DCS flag 807, and the VSflag 808 are each 1 bit long, the packet type field 804 and symmetryfield are each 2 bits long, the CU field 809 and ARQ field are each 3bits long, and the slot pointer field 810 and header HCF field 812 areeach 4 bits long, for a total of 23 bits. A twenty-fourth bit of theheader 801 is used for the purpose of assisting establishment of the RFlink.

[0106] The B/M indicator flag 802 indicates whether the originator ofthe message is a user station 102 or the base station 104. The E flag803 is used to indicate whether or not an extended protocol is in use.The packet type field 804 specifies which of four packet types is beingused, according to Table 8-1A below. TABLE 8-1A Packet Field Packet Type00 Normal traffic 01 Specific poll 10 Control (signaling) traffic 11General poll, or general response

[0107] The packet type field 804 also provides an indication of theusage of the D-field 557, according to Table 8-1B below. TABLE 8-1BPacket Field D-Field Usage 00 D-Channel 01 Correlative ID 10 CorrelativeID 11 Reserved

[0108] The PWR field 805 is a serialized bit stream from the basestation 104 to the user station 102 allowing control of the power levelof the user station 102 transmitter. As each base-to-user message isreceived at the user station 102, the PWR bit from the last message isanalyzed along with the current PWR bit to determine if the power levelof the user station 102 transmitter should be raised, lowered or remainunchanged. Power control action therefore requires that at least twoconsecutive base-to-user messages be received by the user station 102before any action is taken. The action taken is dictated according toTable 8-2 appearing below. TABLE 8-2 Last Bit Current Bit Action 0 0Decrease transmitter power 1 1 Increase transmitter power 0 1 Leavepower unchanged 1 0 Leave power unchanged missing any Leave powerunchanged any missing Leave power unchanged

[0109] The amount of power increase or decrease carried out in responseto receiving commands in the PWR field 805 may be a fixed or presetamount—e.g., 1 dB for each time frame 301 (or more frequently if theuser station 102 is transmitting in multiple time slots 302 per timeframe 301). Using only a single bit for the PWR field 805 saves space inthe header 553 of the base-to-user message. The quality metricsgenerally provide sufficient feedback to allow small power adjustmentsteps over time, but not sufficient feedback to have confidence inmaking substantial power adjustment steps. However, because user stationtransmissions are separated by time within the general geographic regionof a particular base station 104, strict power control of the userstations 102 is not required to avoid intracell or intercellinterference as it typically is with CDMA systems not employing timedivision techniques.

[0110] The symmetry field 806 is used by the base station 104 to grantbandwidth to the user station 102. The bandwidth grant applies to thenext time slot 302 (or 618) in the channel. The symmetry field 806contents may be interpreted according to Table 8-3 below. TABLE 8-3Symmetry Bits Meaning 00 Symmetric bandwidth grant. Each direction hasbeen granted one half of the bandwidth. 01 The maximum bandwidth hasbeen granted to the user station 102, and the minimum bandwidth has beengranted to the base station 104. 10 The maximum bandwidth has beengranted to the base station 104, and the minimum bandwidth has beengranted to the user station 102. 11 Broadcast mode. The entire bandwidthhas been granted to the base station 104. There is no user station 102packet.

[0111] The DCS flag 807 indicates the usage of the D-channel for thecurrent message. The DCS flag 807 is set to one value to indicate thatthe D-channel is disabled to reserve it for use by the application usingthe bearer channel (B-channel), and is set to another value to indicatethat the D-channel is enabled for other usage. The VS flag 808 indicateswhether the base station 104 is using a virtual slot mode. If thevirtual slot mode is active (e.g., the time slot structure of FIG. 6 isused), then all user station 102 transmissions occur one time slotearlier than if the VS mode is inactive.

[0112] The CU field 809 indicates the relative slot utilization for thebase station 104. In a preferred embodiment, the CU field contents aredefined according to Table 8-4 below. TABLE 8-4 CU Field ContentsUtilization 000 No channels available: Find another base station 001 Onechannel available: 911 calls only 010 Two channels available: 911 callsor handover only 011 Few channels available: Class control is in effectfor registrations and originations 100 Nearly full: Access Unrestricted101 Moderately full: Access Unrestricted 110 Partially full: AccessUnrestricted 111 All slots available: Access Unrestricted

[0113] Where class control is in effect for registrations and calloriginations, access leveling and load leveling classes may beidentified in the facility field 707 of the general poll message (seeFIG. 7A).

[0114] The slot pointer field 810 contains an index which identifies thenext time slot to be used in the current base/user packet exchange. Theuser station 102 transmits in the time slot indicated by the slotpointer to continue the exchange. In a particular embodiment, thecontents of the slot pointer field 810 may take on any of sixteendifferent values (e.g., binary 0 to 15), with each value indicating adifferent relative number of time slots from the present time slot inwhich the user station 102 is to transmit. For example, a value of zeromeans that the user station 102 is to transmit in the same slot (in thenext frame if at a regular bandwidth rate, or several frames in thefuture if using a sub-frame rate). A value of one means that the userstation 102 is to transmit in the next time slot of the present timeframe. A value of two means that the user station 102 is to transmit inthe time slot two places ahead in the present time frame, and so on.Examples of operation using slot pointers are described further below.

[0115] The ARQ field 811 allows the receiving entity (either basestation 104 or user station 102) to correct a message error. The ARQfield 811 comprises three subfields of one bit each: (1) an “ARQrequired” sub-field that indicates whether or not ARQ is required forthe message sent; (2) an “ACK” sub-field indicating whether or not thesender of the message received correctly the last message sent; and (3)a “message number” sub-field, which indicates the message number (zeroor one) of the current message. The ACK sub-field and message numbersub-field are always used regardless of whether the ARQ required bit isset.

[0116] If ARQ is required (as determined by the value of the ARQrequired bit), then the receiving entity performs the following steps:

[0117] (1) Compares the message number sub-field of the received messagewith the message-number sub-field of the previously received message; ifthey are the same, the new message is ignored.

[0118] (2) Checks the ACK sub-field of the received message. If thevalue is NAK (indicating that the sender of the message did not receivethe last message correctly), then the receiving entity resends the olddata message; otherwise, it sends a new data message.

[0119] (3) Complements the message number sub-field bit each time a newdata message is sent.

[0120] (4) If a message is received with a FCW error (as explained withrespect to FIG. 7A), or did not receive a message at all, then thereceiving entity sends its data message with the ACK sub-field set toNAK.

[0121] The header HCF field 812 is used for a cyclic redundancy checkcalculated over the preceding bits of the message header.

[0122]FIG. 8B is a diagram of a polling message header format for a pollresponse message (such as general poll response 404 or specific pollresponse 405 of FIG. 4). The polling response header 820 comprises abase/mobile indicator (B/M) flag 821, an extended protocol (E) flag 822,a packet type field 823, a PWR field 824, a symmetry field 825, a DCSflag 826, a spare field 827, an ARQ field 828, and a header framecontrol word (HCF) field 829. In a preferred embodiment, the B/Mindicator flag 821, E flag 822, and DCS flag 826 are each 1 bit long,the packet type field 823, symmetry field 825, and spare field 827 areeach 2 bits long, the ARQ field 828 is 3 bits long, and the HCF field829 is 4 bits long, for a total of 17 bits.

[0123] The B/M indicator flag 821, E flag 822, packet type field 823,PWR field 824, DCS flag 826, ARQ field 828 and HCF field 829 are usedfor the same purposes as their counterpart fields in the base stationheader shown in FIG. 8A. The contents of the symmetry field 825 in theuser station 102 header may be interpreted according to Table 8-5 below.TABLE 8-5 Symmetry Field Meaning 00 Symmetric bandwidth is requested forthe next time slot 01 Maximum bandwidth is requested for the next timeslot 10, 11 (Not presently used)

[0124] In one embodiment in accordance with the header formats of FIGS.8A and 8B, the message headers shown in Table 8-6 correspond to themessage types shown (where “1” and “0” are bit values, and “X” is a bitvalue that is irrelevant or depends upon the application and/or systemstatus). TABLE 8-6 Message Type Header Contents BS General Poll 1X11XXXX XXXX XXXX XXXX XXX BS Specific Poll 1X01 XXXX XXXX XXXX XXXX XXX BSControl Traffic 1X10 XXXX XXXX XXXX XXXX XXX BS Traffic Message 1X00XXXX XXXX XXXX XXXX XXX MS General Response 0X11 XXXX XXXX XXXX X MSSpecific Response 0X01 XXXX XXXX XXXX X MS Control Traffic 0X10 XXXXXXXX XXXX X MS Traffic Message 0X00 XXXX XXXX XXXX X

[0125]FIG. 13A is a diagram of a base station information packet showingin octet format fields generally depicted in FIGS. 5B and 8A. FIG. 13Bis a diagram of a user station information packet showing in octetformat fields generally depicted in FIGS. 5C and 8B.

[0126] Data may be transmitted between the base station 104 and userstations 102 using an M-ary spread spectrum technique. Suitable M-aryspread spectrum transmission and reception techniques are described in,e.g., U.S. Pat. No. 5,022,047 and in U.S. Pat. No. 5,692,007, both ofwhich are assigned to the assignee of the present invention, and both ofwhich are hereby incorporated by reference as if set forth fully herein.In a preferred embodiment, the base station 104 and user stations 102each transmit M-ary direct sequence spread spectrum signals using spreadspectrum codes (called “symbol codes”) of 32 chips. Preferably, N databits are transmitted per symbol code, with M different symbol codes areused to represent up to M different data symbols, where M=log₂ N. In apreferred embodiment, thirty-two different symbol codes are used torepresent thirty-two two different data symbols, each comprising fivebits of data, and differential phase encoding is used to allowtransmission of a 6th bit of data for each symbol code. Techniques ofphase encoding for transmission of an additional bit of information persymbol code are described in, e.g., U.S. Pat. No. 5,692,007 referred toabove.

[0127] Because the base header field 553 is positioned first in the basetransmit data frame 551, it. “loses” the first bit from the firsttransmitted data symbol (which is transmitted using a differentialencoding technique) because it is used as a phase reference bit. Thusthe base header field 553, which comprises four data symbols, is 23 bitsin length. The first data symbol comprises five data bits, and thelatter three data symbols each comprises six data bits. Likewise,because the user header field 523 is positioned first in the usertransmit data frame 521, it “loses” the first bit from the firsttransmitted data symbol because it is used as a phase reference bit.Thus the user header field 523, which comprises three symbols, is 17bits in length. The first data symbol comprises five data bits, and thelatter two data symbols each comprises six data bits.

[0128] Signaling messages (i.e., messages used for control traffic) maybe used to assist in acquisition and maintenance of a channel from thenetwork. Over-the-air signaling messages may commence with a “messagetype” data element located in a message type field. The message typedata element defines the format of the rest of the message, and acts asan operation code to the destination unit (either user station 102 orbase station 104). Exemplary message types for over-the-air signaling(i.e., control traffic) messages appear in Table 9-1 below. TABLE 9-1ACK Acknowledge ANS Answer Incoming Call AUT Authentication Request AURAuthentication Response BAI Base Assist Information CIP Set Cipher ModeCNC Call Connected CSC Circuit Switch Complete DRG De-registrationRequest DRP Drop Incoming Connection HLD Hold ORH Originating HandoverRequest ORG Originate Call RCP Registration Complete RRQ RegistrationRequest SET Set Services SPR Specific Response SYN Synchronize THRTarget handover Request TRA Transport Message

[0129] The number of bits of the message type data element used toidentify the type of message depends mainly upon the number of controltraffic message supported by the system. In a preferred embodiment, themessage type is 8 bits in length. Additional information needed toprocess or act upon the message may be contained in other fields in thesignaling message.

[0130] Messages exchanged between the base station 104 and base stationcontroller 105 or other network entities can be mapped to a local orinternal format referred to as “Notes”. Some of these Notes may resemblethe over-the-air signaling messages exchanged between the base station104 and the user station 102, in order to expedite processing of thecontrol traffic messages. The base station controller 105 may act as aprotocol interface whereby signaling messages are translated to a formcompatible with the mobile switching center 112 and/or network.

[0131] The general content of certain over-the-air signaling messagesthat play a role in handover and related functions are set forth in thetables appearing below. The message content may be viewed as an aspectof “layer three” protocol architecture. TABLE 10-1 Hold (CT-HLD)Information Element Length in Bits Message Type 8 Reserved 152

[0132] Hold (CT-HLD)control traffic messages can be transmitted eitherby the base station 104 or the user station 102. They are generally partof a larger signaling traffic exchange. The user station 102 sends aCT-HLD control traffic message to the base station 104 when the userstation 102 requires more time to process data and return a result tothe base station 104, or when responding to a CT-HLD control trafficmessage from the base station 104. TABLE 10-2 Acknowledge (CT-ACK)Information Element Length in Bits Message Type 8 ACK Response 8 Ack'dCommand 8 Ack State 8 Reserved 128

[0133] Acknowledge (CT-ACK) control traffic messages can be transmittedby either the base station 104 or the user station 102. It is notnecessary the every exchange of control traffic messages end with aCT-ACK message.

[0134] The Ack Response information element of the CT-ACK messagecontains an acknowledgment response indicator. One of two binary values(i.e., a “0” bit) indicates success, while the other of the two binaryvalues (i.e., a “1” bit) indicates failure. The Ack'd Commandinformation element contains the Message Type of the specific commandbeing acknowledged. The Ack State information element contains thecurrent state of the system element (i.e., the base station 104 or userstation 102) which is transmitting the acknowledge. TABLE 10-3 SetCipher Mode (CT-CIP) Information Element Length in Bits Message Type 8Cipher Type 8 Cipher Mode 8 Initialization Vector 64 Cause Type 8 Cause8 Reserved 56

[0135] A Set Cipher Mode (CT-CIP) control traffic message is transmittedfrom the base station 104 to the user station 102 to pass pertinentciphering information to the user station 102 and to instruct the userstation 102 to go into or out of ciphering mode. When the user station102 receives the CT-CIP message, the user station 102 uses the ciphermode parameters to set its ciphering equipment and then switches into orout of ciphering mode. All traffic after the switch to cipher mode willbe ciphered.

[0136] The Cipher Type information element of the CT-CIP messageindicates the type of encryption to be used by the system (e.g., eitherDCS-1900 or Bellcore “C”, for example). The Cipher Mode informationelement indicates the encryption mode being requested by the system. TheInitialization Vector information element contains a value to be used inconjunction with other keying information to initialize the encryptionequipment. The Cause information element consists of eight bits of anencoded parameter indicative of what the cause is of an action, and isspecific to a particular control traffic message. For the CT-CIPmessage, the Cause information field can be set to contain a codeindicating such things as set/change cipher or synchronize cipher. TheCause Type information element defines the cause code set to be returnedwhen either the base station 104 or the user station 102 drops aconnection. The Cause Type is stored as an encoded value that identifiesthe code set of the supporting infrastructure. For example, the CauseType information field can be set to contain a value indicating the useof DCS 1900 cause codes or a value indicating the use of BellcoreGeneric “C” cause codes. TABLE 10-4 Call Origination (CT-ORG)Information Element Length in Bits Message Type 8 Service Request 32 KeySequence Number 8 Class 16 CREF 8 Reserved 88

[0137] The user station 102 sends a Call Originate (CT-ORG)controltraffic message to the base station 104 to request the placement of anoutgoing call.

[0138] The Service Request information element of the CT-ORG messageindicates such things as data versus voice service, use of CRC and ARQ,symmetry or asymmetry of the channel, whether service resources arebeing requested, and frame rate, for example. The Key Sequence Numberinformation element is used to generate a communication key in both thebase station 104 and the user station 102 without having to explicitlypass the key over the air. The Class information element specifies someof the operational parameters of the particular type of user station102. The Class information element can be broken down into sub-fields ofClass Type and Class Information. The Class Type sub-field may indicatethe general class of the user station 102 (e.g., DCS1900 or IS-41),while the Class Information sub-field may indicate such things asprotocol or revision level, encryption algorithm, RF power rating, powerclass, continuous or discontinuous transmission, and licensed orunlicensed bandwidth. The Call Reference (“CREF”) information elementspecifies the circuit to which data in a transport message belongs. TheCREF field corresponds to the ISDN Call Reference information element.The CREF information element may contain a value indicating whether thecircuit is, for example, ISDN, DCS1900 or DECT. TABLE 10-5 Call Connect(CT-CNC) Information Element Length in Bits Message Type 8 ConnectionNumber 40 Map Type 8 Map 32 Cause Type 8 Cause 8 CREF 8 Reserved 48

[0139] The Call Connect (CT-CNC) control traffic message may be sentfrom the base station 104 to the user station 102 when a call, eitherincoming or outgoing, is completed or when an outgoing call from theuser station 102 is rejected.

[0140] The Connection Number information element of the Call Connectmessage specifies the specific network connection which was allocated tocarry the bearer channel of the particular user station 102 from thebase station 104 to the network. Unused nibbles and octets of thisinformation element are filled with “F” hex. The Map information elementdescribes the mapping of time slots to a particular user station 102.The format of the Map element is dependent upon the Map Type informationelement in the same frame. The Map Type information element indicates ifthe frame is a “superframe” (aggregated time slots) or “subframe”(single time slot occurring every N time frames). If a superframe maptype, then each bit in the Map information element corresponds to achannel relative to the current channel. If a subframe map type, the Mapinformation element indicates such things as the submultiplex rate(i.e., the number of frames skipped between transmissions), the framephase (i.e., the number of frames skipped before the firsttransmission), and the channel phase (i.e., the number of time slots orchannels skipped before the first transmission). The Cause and CauseType information elements are as described with respect to the CT-CIPmessage. However, for the CT-CNC message, the Cause information elementindicates whether or not the requested connection has been connected.The CREF information element is the same as described with respect tothe CT-ORG message. TABLE 10-6 Target Handover Request (CT-THR)Information Element Length in Bits Message Type 8 Old Connection Number40 Service Request 32 Key Sequence Number 8 Class 16 Old Base Station ID32 Old Mobility Country Code (MCC) 16 Old Mobility Network Code (MNC) 8

[0141] The Target Handover Request (CT-THR) control traffic message issent from the user station 102 to the target base station 104 toinitiate a terminating handover procedure.

[0142] The Old Connection Number information element of the CT-THRmessage specifies the specific network connection which was allocated tocarry the bearer channel of the user station 102 from the old basestation 104 to the network. Unused nibbles and octets of thisinformation element are filled with “F” hex. The Service Request, KeySequence Number and Class information elements are as described withrespect to the CT-ORG message. The Old Base Station ID informationelement identifies the originating base station 104 in a handover. TheOld MCC information element indicates the mobility country code of theoriginating base station in a handover, and the Old MNC informationelement indicates the mobility network code of the originating basestation in the handover. TABLE 10-7 Originating Handover Request(CT-OHR) Information Element Length in Bits Message Type 8 Base ID 32Mobility Country Code (MCC) 16 Mobility Network Code (MNC) 8 Reserved 56

[0143] The Originating Handover Request (CT-OHR) control traffic messageis sent from the user station 102 to the current base station 104 toinitiate an originating handover procedure.

[0144] The Base ID information element uniquely identifies the targetbase station 104. The MCC and MNC information elements indicate themobility country code and the mobility network code, respectively, ofthe target base station 104. TABLE 10-8 Circuit Switch Complete (CT-CSC)Information Element Length in Bits Message Type 8 Handover Reference 48Map Type 8 Map 32 Reserved 56

[0145] The Circuit Switch Complete (CT-CSC) control traffic message issent from the old base station 104 to the user station 102 to signalthat the network connection is available at the target base station 104.When sent from the old base station 104, the Map information elementwill be all zeroes to indicate that there are no longer any slots on theold base station 104 for the user station 102 to utilize.

[0146] The Handover Reference information element is used to identify aspecific handover process that has already been initiated by anoriginating handover request sequence. In a DCS1900 infrastructuresystem, the handover reference number is assigned by the terminated basestation controller 105. The Map Type and Map information elements are asdescribed with respect to the CT-CNC message. TABLE 10-9 TerminatingHandover Complete (CT-THC) Information Element Length in Bits MessageType 8 Service Request 32 Key Sequence Number 8 Class 16 HandoverReference Number 48 Reserved 48

[0147] A Terminating Handover Complete (CT-THC) control traffic messageis sent by the user station 102 to the target base station 104 toinitiate a terminating handover procedure.

[0148] The Service Request, Key Sequence Number, and Class informationelements are as described for the CT-ORG message. The Handover ReferenceNumber information element is as described for the CT-CSC message. TABLE10-10 Specific Response (CT-SPR) Information Element Length in BitsMessage Type 8 Cipher Type 8 Cipher Mode 8 Key Info 64 Class 16 Reserved56

[0149] The Specific Response (CT-SPR) control traffic message is sentfrom the user station 102 to the base station 104 when the user station102 is listening for a page and receives a Specific Poll control trafficmessage which contains the user station's PID and which is marked as a“paging” Specific Poll message.

[0150] The Cipher Type and Cipher Mode information elements are asdescribed for the CT-CIP message. The Key Info information elementcontains a value to be used in conjunction with other keying informationto initialize the encryption equipment, and the contents of this fielddepend upon the specific type of supporting infrastructure (e.g.,DCS1900). The Class information element is as described for the CT-ORGmessage. TABLE 10-11 Set Service (CT-SET) (user to base) InformationElement Length in Bits Message Type 8 Reserved 80 Map Type 8 Map 32Service Request 32

[0151] The user station 102 sends a Set Service (CT-SET) control trafficmessage to the base station 104 when the user station 102 desires tochange the characteristics of the over-the-air service.

[0152] The Map Type and Map information elements are as described forthe CT-CNC message. The Service Request information element is asdescribed with respect to the CT-ORG message. TABLE 10-12 Set Service(CT-SET) (base to user) Information Element Length in Bits Message Type8 Cause Type 8 Cause 8 Connect Number 40 Reserved 24 Map Type 8 Map 32Service Request 32

[0153] The base station 104 sends a Set Service (CT-SET) control trafficmessage to the user station 102 when the base station 104 wishes tochanges the characteristics of over-the-air service.

[0154] The Connection Number, Map Type and Map information elements ofthe CT-SET message are as described for the CT-CNC message. The CauseType and Cause information elements are as described for the CT-CIPmessage. However, the Cause information element for the CT-SET messageindicates whether the link was successfully established or else failed.TABLE 10-13 Release (CT-REL) Information Element Length in Bits MessageType 8 Cause Type 8 Cause 8 Reserved 136

[0155] The Release (CT-REL)control traffic message is sent by the basestation 104 to the user station 102 when the network releases theconnection in progress or during link setup. The Cause Type and Causeinformation elements are as described for the CT-CIP message. However,the Cause information element for the CT-REL message indicates whetherthe release was initiated by the network, or whether an authenticationrejection occurred. TABLE 10-14 Base Assist (CT-BAM) Information ElementLength in Bits Message Type 8 Base Assist Information 152

[0156] The Base Assist (CT-BAM) control traffic message is sent by thebase station 104 to the user station 102 whenever the base station 104desires to pass information to the user station 102 which will help theuser station 102 in making well informed decisions. The contents of theBase Assist information element vary depending upon the circumstances.TABLE 10-15 Transport (CT-TRA) Information Element Length in BitsMessage Type 8 Transport Data 56

[0157] The Transport (CT-TRA) control traffic message is used fortransporting data between the base station 104 and the user station 102on the circuit specified by the Call Reference Number (CREF). Thecontents of the Transport Data information element varies depending uponthe application, and generally constitutes application level data.

[0158] Transport control traffic messages differ from other controltraffic messages in that the Message Type information element containsadditional information. The format of the Message Type field forTransport messages is as follows: TABLE 10-15A Message Type Header forTransport Header Element Bit Position(s) Transport Bit 8 ACK/NAK 7Message Number 6 CREF 1-5

[0159] The Transport Bit indicates whether or not the message is aTransport message. The ACK/NAK bit indicates whether or not the senderreceived the last message without error. The Message Number bitindicates the message number (0 or 1) of the current message, and shouldalternate for each message sent by the same entity. The Call Referenceidentifies the call.

[0160] The values passed as part of Message Type information elementallow the receiving entity (base station 104 or user station 102) tocorrect a message error. In one embodiment, the following steps areundertaken to attempt to correct a message:

[0161] 1) The receiving entity compares the Message Number of thereceived message with the Message Number of the previously receivedmessage. If they are the same, the receiving entity ignores the newmessage.

[0162] 2) The receiving entity checks the ACK/NAK field of the receivedmessage. If the value is NAK, it resends the old packet, and if thevalue is ACK, it sends the new packet.

[0163] 3) Each sender complements the message number each time a newpacket is sent.

[0164] 4) If the receiving entity receives a message with a FCW error,or if it does not receive a message at all, it resends the old packetwith the NAK bit set.

[0165] In addition to the above messages, various signaling messages maybe used between the base station and the network to convey informationat the call control entity level. Exemplary call control messagesinclude those appearing in Table 9-2 below. TABLE 9-2 Call EstablishmentMessages Direction CC-SETUP Both CC-INFOrmation Both CC-CALL-PROCeedingNetwork -> User CC-ALERTING Both CC-PROGress Network -> User CC-CONNECTBoth CC-CONNECT-ACKnowledge Both CC-EMERGENCY-SETUP User -> NetworkCC-CALL-CONFIRMED User -> Network Call Release Messages DirectionCC-DISConnect Both CC-RELEASE Both CC-RELEASE-COMplete Both Call RelatedSupplementary Services Direction HOLD User -> Network HOLD-ACKnowledgeNetwork -> User HOLD-REJECT Network -> User RETRIEVE User -> NetworkRETRIEVE-ACKnowledge Network -> User RETRIEVE-REJECT Network -> UserDTMF Interaction Direction Start-DTMF User -> Network Stop-DTMF User ->Network Start-DTMF-ACK Network -> User Stop-DTMF-Ack Network -> UserStart-DTMF-Reject Network -> User

[0166] The interplay among the various entities involved in the transferof signaling messages and other information may be better understood byreference to FIG. 21, which depicts a preferred system protocolarchitecture. As illustrated in FIG. 21, a preferred user station 102(designated “MS” in FIG. 21) includes a Communication Management (“CM”)entity, a Mobility Management (“MM”) entity, and a Radio Resources(“RR”) entity, among others. The CM and MM entities of the user station102 communicate with their counterparts at a mobile switching center 112(designated “MSC” in FIG. 21), via links connected across a base station104 (designated “BS” in FIG. 21) and base station controller 105(designated “BSC” in FIG. 21). The various types of signaling interfacesof a preferred embodiment are shown in FIG. 21 by the arrows connectinglike entities.

[0167] The “Layer 3” protocol exchange between the mobile switchingcenter 112 and the base station controller 105 is characterized by theBSSMAP protocol. The “Layer 3” protocol exchange between the mobileswitching center 112 and the user station 102 is characterized by theDirect Transfer Application Part (DTAP). DTAP is further divided intotwo logical sublayers, defined by the CM and MM entities describedabove. The CM includes call control and supplementary servicesmanagement, including short message service.

[0168] Most DTAP messages are not interpreted by the base stationcontroller 105 or the base station 104. Rather, they are transferred tothe network by the mobile switching center 112 over a network interface(such as the GSM A-interface) Most radio resource (RR) messages aremapped to BSSMAP messages at the base station controller 112. However,some of these messages are interpreted by the base station 104 (e.g.,paging messages). The control management (CM) part of the protocol isaddressed by an ISDN based CM message set, referred to as IGCC (ISDNGeneric Call Control). Control management messages from the user station102 are directly transferred to the network over the interface at themobile switching center 112. Interface adapters at the user station 102and the base station controller 105 segment control management (i.e.,IGCC) messages into packets, which are individually transported betweenthe user station 102 and the base station 104 via CT-TRA Control Trafficmessages and between the base station 104 and base station controller105 via Transport Notes. Notes are transmitted over a CCITT ISDN datalink (Q.920/Q.921) The interface adapters at the user station 102 andbase station controller 105 are responsible for ensuring that thepackets are sequenced properly and the entire IGCC message is errorfree.

[0169] Radio resource (RR) messages and mobility management (MM)messages take the form of internal Notes between the base stationcontroller 105 and base station 104, and are mapped at the base stationto over-the-air messages when sent to the user station 102.

[0170] Exemplary message flow diagrams for various calling functions areshown in FIGS. 9, 10, 11A-11C and 12A-12B. While generally describedwith respect to features referenced in the FIG. 3 embodiment, they haveequal applicability to the FIG. 6 embodiment.

[0171] An exemplary message flow diagram for call origination from auser station 102 is shown in FIG. 9. In FIG. 9 messages are designatedby arrows (1) between a user station 102 (abbreviated “MS”) and a basestation 104 (abbreviated “BS”), (2) between the base station 104 and abase station controller 105 (abbreviated “BSC”), and (3) between thebase station controller 105 and a mobile switching center 112(abbreviated “MSC”). The MSC 112 generally acts a switch controllingaccess to the network 106 (as shown, e.g., in FIG. 2). Control trafficmessages between the user station 102 and the base station 104 aretypically preceded by the initials “CT” in FIG. 9. The steps numbered 1through 17 associated with the arrows appearing in FIG. 9 are explainedbelow:

1. A method comprising transmitting a plurality of base-to-user messages from a base station to a user station within time slots of a single time frame, each time frame having a plurality of duplex time slots, and each time slot having a base transmission interval and a user transmission interval, the base-to-user messages alternating with one or more user-to-base messages, each of the base-to-user messages having an information element indicating a location of a subsequent time slot available for communication.
 2. The method of claim 1, wherein the plurality of base-to-user and user-to-base messages comprise control traffic messages, and wherein transmitting the plurality of base-to-user messages comprises completing a control traffic transaction.
 3. The method of claim 1, further comprising completing a handshake transaction between the base station and the user station so as to establish a communication channel and exchanging bearer traffic messages between the user station and the base station over the established communication channel.
 4. The method of claim 1, wherein transmitting the plurality of base-to-user messages comprises spread spectrum encoding the base-to-user messages.
 5. The method of claim 1, wherein the user-to-base messages are each in the respective time slot indicated by the information element of the preceding base-to-user message from the base station.
 6. The method of claim 1, wherein the duplex time slots comprise virtual time slots.
 7. The method of claim 1, wherein the information element comprises a numerical value indicating a relative number of time slots until the subsequent time slot.
 8. The method of claim 1, wherein the information element indicates a location of a subsequent time slot available to a user station for communication.
 9. The method of claim 1, wherein the base-to-user messages comprise at least one general poll message broadcast to user stations and at least one specific poll message directed to a specific user station, and wherein the user-to-base messages comprises at least one general response message transmitted by a user station in response to the at least one general poll message.
 10. A message structure comprising: a data segment within a time slot of a time frame wherein each time frame comprises a plurality of duplex time slots, and each time slot comprises a base transmission interval and a user transmission interval; and a header segment within the time slot and adjoining the data segment, the header segment including a next slot pointer to indicate a subsequent available time slot for communication by a message recipient, the subsequent time slot being designated without regard to whether the time slot is in the same time frame or not.
 11. The message structure of claim 10, wherein the next slot pointer comprises a numerical value indicating a relative number of time slots until the subsequent time slot is available for communication.
 12. The message structure of claim 10, wherein the next slot pointer comprises a numerical value indicating an absolute position of the subsequent time slot available for communication relative to a starting point of a time frame.
 13. The message structure of claim 10, wherein the duplex time slots are virtual time slots.
 14. A method comprising: transmitting a first plurality of control traffic messages from a user station to a base station in a user transmission interval of a first plurality of time slots, at least two of the first plurality of time slots being within a single time frame; receiving a second plurality of control traffic messages from the base station at the user station in a base transmission interval of a second plurality of time slots, at least one of the second plurality of control traffic messages comprising a next slot pointer indicating to the user station an available time slot for transmitting one of the first plurality of control traffic messages, at least two of the second plurality of time slots being within the single time frame.
 15. The method of claim 14 wherein transmitting the plurality of control traffic messages comprises spread spectrum encoding the first plurality of control traffic message, and wherein transmitting the second plurality of control traffic messages comprises spread spectrum encoding the second plurality of control traffic messages.
 16. The method of clam 14 wherein the first plurality of control traffic messages and the second plurality of control traffic messages are transmitted over the same frequency band.
 17. The method of claim 14, further comprising: completing a handshake transaction between the base station and a user station using the first and second plurality of control traffic messages so as to establish a communication channel; and exchanging bearer traffic messages between the user station and the base station over the established communication channel.
 18. The method of claim 14, further comprising transmitting a second plurality of user-to-base messages from the user station to the base station, each of the plurality of user-to-base messages transmitted in the respective time slot indicated by the information element of the preceding base-to-user control traffic message from the base station.
 19. The method of claim 14, wherein the plurality of time slots are virtual time slots.
 20. The method of claim 14, wherein the plurality of time slots are virtual time slots, such that the user transmission interval of a time slot is not adjacent in time to the base transmission interval of the time slot.
 21. The method of claim 14, wherein at least two of the base station control traffic transmissions occurring within the timespan of a single time frame, separated by at least one of the user station control traffic transmissions, the base station control traffic transmissions each containing an information element pointing to a previously unassigned time slot, and the user station control traffic transmissions are each transmitted in the time slot identified by the information element in the immediately previous base station control traffic transmission.
 22. A multiple-user communication system, comprising: a base station to generate a series of time frames, each of the time frames comprising a plurality of time slots each having a base transmission interval and a user transmission interval; wherein the base station transmits control traffic messages during selected ones of the time slots, each control traffic message comprising a next slot pointer identifying a subsequent time slot available to a user station for communication; wherein the base station receives in the time slot identified by the next slot pointer of that control traffic message from a user station responding to one of the control traffic messages; and wherein the base station exchange at least three control traffic messages in alternating succession with the responding user station and within the timespan of a single time frame.
 23. The multiple-user system of claim 22 wherein base station transmissions are transmitted in a spread spectrum format.
 24. The multiple-user system of claim 22 wherein each of the control traffic messages transmitted by the base station comprise a base header segment, and wherein the next slot point is contained within the base header segment.
 25. The multiple-user communication system of claim 22, wherein the plurality of time slots are virtual time slots.
 26. A multiple-user wireless communication system, comprising: a series of time frames each divided into a plurality of time slots collectively comprising a plurality of user transmission intervals and a plurality of base transmission intervals, each of the time slots comprising a user transmission interval followed by a base transmission interval such that the user transmission intervals alternate with the base transmission intervals within each time frame; and a user station; wherein the user station receives, over a designated frequency band, one or more base-to-user control traffic messages, each of the base-to-user control traffic messages transmitted in the base transmission interval of one of the time slots, at least one of the base-to-user control traffic messages comprising an information element indicating a location of a subsequent time slot available for communication; and wherein the user station transmits in response to the base-to-user control traffic messages, over the designated frequency band, a user-to-base control traffic message to the base station, in the user transmission interval of one of the time slots indicated by the information element of the preceding base-to-user control traffic message from the base station.
 27. The multiple user wireless communication system of claim 26, wherein the base-to-user control traffic messages comprise at least one general poll message broadcast to user stations and at least one specific poll message directed to the user station, and wherein the user-to-base control traffic message comprises at least one general response message transmitted by the user station in response to the at least one general poll message.
 28. The multiple user wireless communication system of claim 26, wherein the user station communicates with the base station in time division duplex within selected time slots through the exchange of data traffic messages.
 29. A method comprising: transmitting a first message to a station, the first message comprising a data segment and an information element indicating a location of a subsequent time slot available for communication; and receiving a second message from the station in the indicated time slot.
 30. The method of claim 29, wherein the station comprises a user station.
 31. The method of claim 56, wherein the first message comprises a base-to-user message and the second message comprises a user-to-base message.
 32. The method of claim 29, wherein the time slot comprises a duplex time slot comprising a forward link transmission interval and a reverse link transmission interval.
 33. The method of claim 29, wherein the duplex time slot comprises a virtual time slot and the forward link transmission interval and the reverse link transmission interval are non-adjacent in time.
 34. The method of claim 29, wherein the first and second messages are transmitted over the same frequency band.
 35. The method of claim 29, wherein the time frame comprises a plurality of duplex time slots. 